Evaporation material for the production of average refractive optical layers

ABSTRACT

The invention relates to a vapor-deposition material for the production of optical layers of medium refractive index which comprises aluminum oxide and gadolinium oxide, dysprosium oxide and/or ytterbium oxide, to a process for the preparation thereof, and to the use thereof.

The invention relates to a vapour-deposition material for the productionof optical layers of medium refractive index which comprises aluminiumoxide and gadolinium oxide, dysprosium oxide and/or ytterbium oxide.

For the protection of the surfaces or in order to achieve certainoptical properties, optical components are generally provided with thincoatings. These optical components include, for example, optical lenses,spectacle lenses, lenses for cameras, binoculars or for other opticalinstruments, beam splitters, prisms, mirrors, window panes and the like.

The coatings can be employed for the treatment of the said surfaces byhardening and/or increasing the chemical resistance to damage bymechanical, chemical or environmental influences. In many cases, the aimof such surface coatings is, however, reduced reflection, which appliesin particular to spectacle lenses, camera lenses and the like. However,there are also applications in which increased reflection is desired oralternatively optical layers which have to have defined refractiveindices or absorption properties, such as, for example, for theproduction of interference mirrors, beam splitters, heat filters orcold-light mirrors. The optimum properties of the layers are generallyset by selecting suitable coating materials, various layer thicknessesand single- or multilayered structures comprising, where appropriate,different materials having different refractive indices. Thus, forexample, in reflection-reducing coatings, it is possible to achieve areduction in the reflection to less than 1% over the entire visibleradiation spectrum.

It is known that the above-mentioned coating layers can be produced bythe application of thin layers of different materials, in particularcomprising oxides, such as SiO₂, TiO₂, ZrO₂, MgO, Al₂O₃, but alsocomprising fluorides, such as MgF₂, and mixtures of these substances.

The coating materials are selected here in accordance with the targetoptical properties and in accordance with the processing properties ofthe materials.

The coating of optical substrates is usually carried out using ahigh-vacuum vapour-deposition process. In this, firstly the substrateand a flask containing the vapour-deposition substance are placed in asuitable high-vacuum vapour-deposition apparatus, the apparatus issubsequently evacuated, and the vapour-deposition substance isevaporated by heating and/or electron beam bombardment, with thevapour-deposition material precipitating on the substrate surface in theform of a thin layer. Corresponding apparatuses and processes areconventional prior art.

Only a limited selection of substances is known which are suitable forthe production of layers of medium refractive index, which generallyhave a refractive index of between 1.6 and 1.9. The starting materialsemployed are, for example, the oxides of aluminium, magnesium, yttrium,lanthanum and praseodymium, but also cerium fluoride, lanthanum fluorideor mixtures thereof.

However, the preferred starting material used for the production oflayers of medium refractive index is aluminium oxide.

Although these materials are suitable per se for the production oflayers of medium refractive index, they have a number of disadvantageswhich make their practical use more difficult.

Thus, these substances have, for example, high melting and boilingpoints, which are frequently also close together. In order to improvethe usability, however, it is necessary for the starting material to bemelted completely before commencement of the evaporation since only inthis way can a uniform and adequate evaporation rate be ensured. This isa prerequisite for the deposition of homogeneous and uniformly thicklayers.

However, magnesium oxide and yttrium oxide do not melt completely underthe usual working conditions and, for example due to the sublimationtendency of yttrium oxide, are difficult to evaporate in full, meaningthat the layers produced therewith generally have variations in layerthickness. With magnesium oxide and lanthanum oxide, vapour depositionis accompanied by the formation of porous layers which take up moistureand thus become unstable. In addition, MgO binds CO₂ from moist air inthe form of carbonate. Cerium fluoride and lanthanum fluoride also forminhomogeneous layers which do not have the requisite hardness anddurability.

For specific applications in which refractive indices of between 1.7 and1.8 are required, aluminium oxide (n=1.63) and yttrium oxide (n=1.85),for example, cannot be employed in pure form.

For this reason, there have been numerous attempts to reduce the meltingpoint of the base materials by means of suitable additives. At the sametime, the refractive index can also be set specifically through theaddition of additives.

However, when selecting the materials to be added, it must be noted thatonly materials which have no or no significant absorption in a broadrange of the radiation spectrum are suitable. Of particular importancehere is freedom from absorption from the near infrared via the visiblespectral region to the near UV wavelength range (up to about 200 nm).For this reason, praseodymium oxide and neodymium oxide, for example,are not suitable as additives since they have absorption maxima in thisrange.

However, the use of mixed systems also proves to be difficult for otherreasons since mixed systems in many cases evaporate incongruently, i.e.their composition changes in the course of the evaporation process.However, the composition of the deposited layers also changes therewith.The inhomogeneous layers formed no longer have a reproducible uniformrefractive index. This problem appears to an increased extent if, asgenerally customary, layer systems comprising a plurality of layers areapplied to a substrate.

U.S. Pat. No. 3,934,961 discloses a three-layered antireflection coatingwhose first layer on the substrate consists of aluminium oxide andzirconium oxide. It is thus possible to set layers of medium refractiveindex between 1.63 and 1.75. Although layers of this type exhibit noabsorption in the visible spectral region, it has been found inpractice, however, that the metal oxides evaporate incongruently andthus result in inhomogeneous layers.

DE-A 42 19 817 describes a vapour-deposition material for the productionof optical layers of medium refractive index which consists of acompound of the formula La_(1−x)Al_(1+x)O₃, where x=0 to 0.84. Thiscompound is prepared from a mixture of lanthanum oxide and aluminiumoxide. However, the lanthanum oxide content results in increasedsensitivity to a moist atmosphere since lanthanum oxide takes upmoisture. Just in order to minimise the associated processing problems,complex measures therefore have to be taken as early as during thepreparation and later during the processing of the vapour-depositionmaterials. However, the layers formed are in any case unstable atincreased atmospheric humidity levels. In addition, lanthanum has anatural radioactive isotope which acts as gamma emitter and can causedefects and damage to sensitive components, for example detectors, inoptical layers produced from the above-mentioned compound. A furtherdisadvantage consists in that the mixture has to be subjected tosintering temperatures of 1600° C. or higher before evaporation, whichcan cause damage to the equipment and apparatuses used, for example tomelting crucibles and heating coils.

According to Japanese laid-open specification JP-A-2000-171609, acompound of the formula Sm_(1−x)Al_(1+x)O₃, where −1<x<1, is used forthe production of layers of medium refractive index. This material doesnot change its composition during the evaporation, but has an absorptionband due to the samarium ion at a wavelength of about 400 nm, whichrestricts use beyond the visible spectral region, in particular in theultraviolet spectral region. In addition, samarium also has naturalradioactive isotopes which occur with a high relative frequency of 15%and, as alpha emitters, likewise have the disadvantages alreadydescribed above. In order to sinter the mixture, temperatures of about1500° C. are necessary.

Further oxides of elements from the lanthanoid group are employed asindividual substances in optical layers.

Thus, for example, U.S. Pat. No. 4,794,607 describes a semiconductorlaser which can have an antireflection layer of gadolinium oxide. A thininterlayer of aluminium oxide increases the adhesion of the gadoliniumoxide layer to the laser. The refractive index that can be achieved withthe gadolinium oxide layer is not described.

DE-A 33 35 557 discloses a synthetic resin lens having a refractiveindex of about 1.6 which may comprise ytterbium oxide in a layer in amultilayered system for reflection reduction. The refractive index thatcan be achieved with this ytterbium oxide layer is not mentioned.Mixtures of ytterbium oxide with other materials are not described.

The object of the present invention was to provide a vapour-depositionmaterial for the production of optical layers of medium refractive indexwhich has high durability, is insensitive to moisture, acids andalkalis, has low radioactivity, is transparent and non-absorbent over abroad spectral range, does not change its original composition duringmelting and evaporation, requires only low sintering temperatures andwith the aid of which layers of medium refractive index having theabove-mentioned properties whose refractive index can be setspecifically in the range between 1.7 and 1.8 can be obtained.

The object according to the invention is achieved by a vapour-depositionmaterial for the production of optical layers of medium refractiveindex, comprising aluminium oxide and at least one compound from thegroup consisting of gadolinium oxide, dysprosium oxide and ytterbiumoxide.

The object according to the invention is also achieved by a process forthe preparation of a vapour-deposition material for the production ofoptical layers of medium refractive index, in which aluminium oxide ismixed with at least one compound from the group consisting of gadoliniumoxide, dysprosium oxide and ytterbium oxide, the mixture is compressedor suspended, shaped and subsequently sintered.

In addition, the present invention relates to the use of avapour-deposition material comprising aluminium oxide and at least onecompound from the group consisting of gadolinium oxide, dysprosium oxideand ytterbium oxide for the production of optical layers of mediumrefractive index.

The vapour-deposition material according to the invention comprisesaluminium oxide and at least one compound from the group consisting ofgadolinium oxide (Gd₂O₃), dysprosium oxide (Dy₂O₃) and ytterbium oxide(Yb₂O₃). The molar composition of the mixture here determines therefractive index of the layer which can be produced by means of thevapour-deposition material. Pure aluminium oxide results in a refractiveindex of the layer of about 1.63. This is too low for certainapplications, in which refractive indices of from 1.7 to 1.8 arenecessary. The refractive indices of pure layers of gadolinium oxide,dysprosium oxide or ytterbium oxide vary, depending on the layerthickness and application method, between 1.55 and 1.85. Addition ofthese substances thus in most cases enables the refractive index to beincreased compared with a layer of pure aluminium oxide.

The molar composition of the mixture is therefore set in accordance withthe requisite refractive index. It can be varied within broad limits andis from 1:99 to 99:1 (mol %) in binary mixtures. If a plurality ofcompounds from the group consisting of gadolinium oxide, dysprosiumoxide and ytterbium oxide is employed, each of the constituents may bepresent in an amount of up to 98 mol % in the case of ternary mixturesand in an amount of up to 97 mol % in the case of quaternary mixtures.

A proportion of from 20 to 80 mol % of aluminium oxide and from 80 to 20mol % of the at least one compound from the group consisting ofgadolinium oxide, dysprosium oxide and ytterbium oxide is particularlyadvantageous here, but in particular a ratio of from 40:60 to 60:40.

Besides reliable setting of the desired refractive index, preferably inthe range between 1.7 and 1.8, this mixture also offers a notinconsiderable economic advantage over pure layers of gadolinium oxide,dysprosium oxide or ytterbium oxide since aluminium oxide is morereadily available and less expensive.

The use of ternary or quaternary mixtures of aluminium oxide andgadolinium oxide and/or dysprosium oxide and/or ytterbium oxide canfavour precise control of the refractive index. The variation latitudeof the medium refractive indices in the range from 1.6 to 1.9 that canbe set is thus increased.

Besides the possibility of being able to produce layers havingrefractive indices in a pre-defined range, the vapour-depositionmaterials according to the invention also have a number of furtheradvantages. Although they are mixtures, they evaporate congruently, i.e.their composition remains virtually unchanged during the evaporationprocess. Homogeneous layers of medium refractive index can thus beproduced reproducibly. This is particularly advantageous if multilayeredsystems with materials of different refractive indices are deposited oneon top of the other. Otherwise, a considerable deviation from the systemvalue originally calculated would arise via the number of layers ofmedium refractive index.

The optical layers obtained are transparent over a broad spectral range,i.e. from about 250 nm to about 7 μm, and have only extremely lowabsorption in this range. In particular in the visible spectral region,they are totally free from absorption. This makes them particularlyinteresting for use in polarising beam splitters and dichroic filters.

The vapour-deposition material according to the invention furthermorehas improved durability, which has a positive effect predominantly in amoist-warm environment. Since the starting materials are stable toatmospheric humidity, they are simple to handle and particularprotective measures do not have to be taken during the preparation andfurther processing of the vapour-deposition materials. Furthermore, theoptical layers produced with these materials are distinguished by highstability in a moist-warm atmosphere and to acids and alkalis.

A further advantage of the vapour-deposition materials of the presentinvention consists in that the substances used have no radioactiveisotopes. Neither the vapour-deposition materials themselves nor thelayers produced therewith therefore emit radioactively, meaning thatsafety measures are unnecessary and damage in this respect to opticalcomponents or detectors which come into contact with the layers is notto be expected.

The vapour-deposition material according to the invention is prepared bya process in which aluminium oxide is mixed with at least one compoundfrom the group consisting of gadolinium oxide, dysprosium oxide andytterbium oxide, the mixture is compressed or suspended, shaped andsubsequently sintered.

The molar mixing ratio of the starting components here is dependent onthe target refractive index of the layer to be applied with the mixtureand can be varied in a broad range, namely from 1:99 to 99:1 mol % inthe case of binary mixtures and with a proportion of up to 98 mol % forone of the oxides in a ternary mixture and up to 97 mol % in the case ofa quaternary mixture.

A proportion of from 20 to 80 mol % of aluminium oxide and from 80 to 20mol % of the at least one compound from the group consisting ofgadolinium oxide, dysprosium oxide and ytterbium oxide is particularlyadvantageous, but in particular a ratio of from 40:60 to 60:40.

The components are mixed intimately with one another and subsequentlycompressed and shaped by means of suitable compression measures knownper se. However, it is also possible to prepare a suspension of themixed components in a suitable carrier medium, which is shaped andsubsequently dried. A suitable carrier medium is, for example, water, towhich, if necessary, binders, such as polyvinyl alcohol, methylcelluloseor polyethylene glycol, and, if desired, assistants, such as, forexample, wetting agents or antifoams, are added. The suspensionoperation is followed by shaping. In this, various known techniques,such as extrusion, injection moulding or spray drying, can be used. Theshapes obtained are dried and freed from the binder, for example byburning out. This is carried out for reasons of better handling andmetering of the mixtures, which thus also become accessible tocontinuous vapour-deposition processes. The shapes into which themixture is converted are therefore not limited. Suitable shapes are allthose which facilitate simple handling and good metering, which play aspecial role, in particular, in the continuous coating of substrateswith the vapour-deposition material according to the invention and thereplenishment process which is necessary for this purpose. Preferredshapes are therefore various tablet shapes, pellets, discs, truncatedcones, grains or granules, rods or also beads.

The shaped mixtures are subsequently sintered. The sintering processhere can be carried out under various conditions. In general, thevapour-deposition material according to the invention is sintered inair. However, the sintering process can also be carried out underreduced pressure or under an inert gas, for example argon. It isparticularly advantageous that the sintering temperatures required arein some cases considerably lower than for other suitablevapour-deposition materials for layers of medium refractive index fromthe prior art. They are from about 1300 to 1600° C. in air or underreduced pressure or under an inert gas. These comparatively lowtemperatures result in lower thermal load of the equipment andapparatuses, such as, for example, melting crucibles and heating coils,meaning that their life is increased.

The shaped sintered products formed remain in their shape duringstorage, transport and introduction into the evaporation apparatus andare stable in their composition during the entire subsequent melting andevaporation process.

After sintering and cooling, the vapour-deposition materials accordingto the invention are ready for use for the production of optical layersof medium refractive index having refractive indices of between 1.6 and1.9.

The vapour-deposition materials according to the present invention canbe used to coat all suitable substrates, which can consist of the knownsuitable materials, such as, for example, various glasses or plastics,and are in the shape of panes, prisms, sheets, shaped substrates, suchas lenses, spectacle lenses, camera lenses or the like. Their nature,size, shape, material and surface quality are not limited and arerestricted only by the usability of the substrates in the coatingapparatus, since the substrates can be introduced into the apparatus andmust remain stable under the temperature and pressure conditionsprevailing therein.

It has proven advantageous to heat the substrates before and during thecoating operation, so that the vapour-deposition material hits apre-heated substrate. However, this measure is known per se from theprior art.

The vapour-deposition process employed is usually a high-vacuumvapour-deposition process in which the vapour-deposition material in asuitable flask, which is known as the evaporation crucible orevaporation boat, is introduced into a vacuum apparatus together withthe substrate to be coated.

The apparatus is subsequently evacuated, and the vapour-depositionmaterial is caused to evaporate by heating and/or electron beambombardment. The vapour-deposition material precipitates on thesubstrate in the form of a thin layer.

During the evaporation, oxygen can be added in order to ensure completeoxidation of the layers. Furthermore, ion bombardment of the substrate(ion assisted deposition, plasma assisted deposition) can be carried outduring the coating operation in order to increase the density of thelayers and in order to improve the adhesion, in particular to unheatedsubstrates.

A plurality of layers is frequently deposited alternately one on top ofthe other on the substrate. Through a suitable choice of the refractiveindices of the individual layers, it is thereby possible to setspecifically the desired optical properties, such as, for example,reflection reduction, reflection increase or the setting of apre-defined refractive index for the system as a whole. However,multilayered arrangements of this type on optical substrates have beenknown per se for some time and are frequently used.

The vapour-deposition material according to the invention can be used toproduce adherent optical layers of medium refractive index on suitablesubstrates which are non-absorbent in a broad spectral range, aretransparent and homogeneous, have a medium refractive index in the rangefrom about 1.6 to about 1.9, are stable in a moist-warm environment andto acids and alkalis and emit no radioactive radiation.

The invention will be explained below by means of a number of examples,but without being restricted thereto.

EXAMPLE 1 Mixture of Al₂O₃ and Dy₂O₃

21.11 g of aluminium oxide (50 mol %) and 77.21 g of dysprosium oxide(50 mol %) are mixed intimately with one another until a homogeneousmixture has formed. This mixture is shaped into tablets, which arecalcined at 1300° C. in air for 4 hours. After cooling, the tablets areintroduced into the crucible of an electron-beam evaporator, for examplea Leybold A700Q unit. Purified substrates of quartz glass and spectaclecrown glass BK7 are introduced into the substrate holder of the unit.The unit is evacuated to a pressure of 2×10⁻³ Pa. The substrates areheated to about 300° C. Oxygen is then admitted into the unit to apressure of 2×10⁻² Pa in order to achieve complete oxidation. The tabletof the vapour-deposition material is subsequently heated to theevaporation temperature of about 2100° C., and a layer having athickness of about 280 nm is vapour-deposited on the substrates. Thelayer thickness is determined using a vibrating quartz layer thicknessmeasuring instrument. After cooling, the unit is flooded with air, andthe coated substrates are removed. The transmission and reflectionspectra are measured using a spectrophotometer, and the layer thicknessand refractive index are calculated therefrom. The layers applied arehomogeneous and have a refractive index of 1.70 at a wavelength of 500nm. The absorption in the range from 300 to 900 nm is determined asbeing less than 1%.

EXAMPLE 2 Mixture of Al₂O₃ and Yb₂O₃

20.11 g of aluminium oxide (50 mol %) and 81.57 g of ytterbium oxide (50mol %) are mixed intimately with one another until a homogeneous mixturehas formed. This mixture is shaped into tablets, which are calcined at1300° C. in air for 4 hours. The cooled tablets are subsequentlyintroduced into the crucible of an electron-beam evaporator, for examplea Leybold A700Q vapour-deposition unit. Purified substrates of quartzglass and spectacle crown glass BK7 are introduced into the substrateholder of the unit. The unit is evacuated to a pressure of 2×10⁻³ Pa.The substrates are heated to about 300° C. Oxygen is then admitted intothe unit to a pressure of 2×10⁻² Pa. The tablet of the vapour-depositionmaterial is subsequently heated to the evaporation temperature of 2100°C., and a layer having a thickness of about 280 nm is vapour-depositedon the substrates. After cooling, the unit is flooded with air, and thecoated substrates are removed. The transmission and reflection spectraare measured using a spectrophotometer, and the layer thickness andrefractive index are calculated therefrom. The layers applied arehomogeneous and have a refractive index of 1.76 at a wavelength of 500nm. The absorption in the range from 300 to 900 nm is determined asbeing less than 1%.

EXAMPLE 3 Mixture of Al₂O₃ and Gd₂O₃

21.56 g of aluminium oxide (50 mol %) and 76.67 g of gadolinium oxide(50 mol %) are mixed intimately with one another until a homogeneousmixture has formed. This mixture is shaped into tablets, which arecalcined at 130° C. in air for 4 hours. After cooling, the tablets areintroduced into the crucible of an electron-beam evaporator in a LeyboldA700Q vapour-deposition unit. Purified substrates of quartz glass andspectacle crown glass BK7 are introduced into the substrate holder ofthe unit. The unit is evacuated to a pressure of 2×10⁻³ Pa. Thesubstrates are heated to about 300° C. Oxygen is then admitted into theunit to a pressure of 2×10⁻² Pa. The tablet of the vapour-depositionmaterial is subsequently heated to the evaporation temperature of about2100° C., and a layer having a thickness of about 240 nm isvapour-deposited on the substrates. After cooling, the unit is floodedwith air, and the coated substrates are removed. The transmission andreflection spectra are measured using a spectro-photometer, and thelayer thickness and refractive index are calculated therefrom. Thelayers applied are homogeneous and have a refractive index of 1.71 at awavelength of 500 nm. The absorption in the range from 300 to 900 nm isdetermined as being less than 1%.

EXAMPLE 4 Mixture of Al₂O₃ and Yb₂O₃

14.46 g of aluminium oxide (40 mol %) and 83.65 g of ytterbium oxide (60mol %) are mixed intimately with one another until a homogeneous mixturehas formed. This mixture is shaped into tablets, which are calcined at1300° C. in air for 4 hours. After cooling, the tablets are introducedinto the crucible of an electron-beam evaporator in a Leybold A700Qvapour-deposition unit. Purified substrates of quartz glass andspectacle crown glass BK7 are introduced into the substrate holder ofthe unit. The unit is evacuated to a pressure of 2×10⁻³ Pa. Thesubstrates are heated to about 300° C. Oxygen is then admitted into theunit to a pressure of 2×10⁻² Pa. The tablet of the vapour-depositionmaterial is subsequently heated to the evaporation temperature of about2100° C., and a layer having a thickness of about 240 nm isvapour-deposited on the substrates. After cooling, the unit is floodedwith air, and the coated substrates are removed. The transmission andreflection spectra are measured using a spectro-photometer, and thelayer thickness and refractive index are calculated therefrom. Thelayers applied are homogeneous and have a refractive index of 1.80 at awavelength of 500 nm. The absorption in the range from 300 to 900 nm isdetermined as being less than 1%.

EXAMPLE 5 Mixture of Al₂O₃, Dy₂O₃ and Gd₂O₃

21.7% by weight (50 mol %) of aluminium oxide, 39.69% by weight (25 mol%) of dysprosium oxide and 38.57% by weight (25 mol %) of gadoliniumoxide are mixed intimately with one another until a homogeneous mixturehas formed. This mixture is shaped into tablets, which are calcined at1500° C. in air for 4 hours. After cooling, the tablets are introducedinto the crucible of an electron-beam evaporator in a Leybold L560vapour-deposition unit. Purified substrates of quartz glass andspectacle crown glass BK7 are introduced into the substrate holder ofthe unit. The unit is evacuated to a pressure of 2×10⁻³ Pa. Thesubstrates are heated to about 250° C. Oxygen is then admitted into theunit to a pressure of 2×10⁻² Pa. The tablet of the vapour-depositionmaterial is subsequently heated to the evaporation temperature of about2100° C., and a layer having a thickness of about 270 nm isvapour-deposited on the substrates. After cooling, the unit is floodedwith air, and the coated substrates are removed. The transmission andreflection spectra are measured using a spectrophotometer, and the layerthickness and refractive index are calculated therefrom. The layersapplied are homogeneous and have a refractive index of 1.72 at awavelength of 500 nm. The absorption in the range from 300 to 900 nm isdetermined as being less than 1%.

EXAMPLE 6 Mixture of Al₂O₃, Dy₂O₃, Gd₂O₃ and Yb₂O₃

21.31% by weight (50 mol %) of aluminium oxide, 25.97% by weight (16.66mol %) of dysprosium oxide, 25.42% by weight (16.66 mol %) of gadoliniumoxide and 27.47% by weight (16.68 mol %) of ytterbium oxide are mixedintimately with one another until a homogeneous mixture has formed. Thismixture is shaped into tablets, which are calcined at 1500° C. in airfor 6 hours. After cooling, the tablets are introduced into the crucibleof an electron-beam evaporator in a Leybold L560 vapour-deposition unit.Purified substrates of quartz glass and spectacle crown glass BK7 areintroduced into the substrate holder of the unit. The unit is evacuatedto a pressure of 2×10⁻³ Pa. The substrates are heated to about 250° C.Oxygen is then admitted into the unit to a pressure of 2×10⁻² Pa. Thetablet of the vapour-deposition material is subsequently heated to theevaporation temperature of about 2100° C., and a layer having athickness of about 290 nm is vapour-deposited on the substrates. Aftercooling, the unit is flooded with air, and the coated substrates areremoved. The transmission and reflection spectra are measured using aspectrophotometer, and the layer thickness and refractive index arecalculated therefrom. The layers applied are homogeneous and have arefractive index of 1.73 at a wavelength of 500 nm. The absorption inthe range from 300 to 900 nm is determined as being less than 1%.

Durability Test

The coated substrates obtained in Examples 1 to 4 were subjected tovarious durability tests. In these, the substrates were stored invarious media under various conditions.

Storage in: Condition: Deionised water 6 hours at room temperatureBoiling deionised water 10 minutes Saline solution 4.5% by weight indeionised water, 6 hours, room temperature Hydrochloric acid solution0.01 molar, 6 hours at room temperature (pH 2) Sodium hydroxide solution0.01 molar, 6 hours at room temperature (pH 12)

Results: detachment of the applied layer is not observed in any of thesample glasses after the respective test. Spots and/or hazing are notevident.

Analysis of the Transmission and Reflection Spectra:

For the coated substrates obtained in Examples 1 to 4 and a comparativesample consisting of a quartz substrate coated with Al₂O₃, thetransmission and reflection spectra were determined after theabove-mentioned durability tests and assessed in comparison with thetransmission and reflection spectra of the untested substrates. Thefollowing results were obtained:

Substance Test Al₂O₃/Yb₂O₃ Al₂O₃/Dy₂O₃ Al₂O₃/Gd₂O₃ Al₂O₃Deionised + + + + water Boiling test + + + − NaCl + + + + solution HClsolution − + ? ? NaOH + + + − solution + spectra unchanged − spectraconsiderably changed ? spectra somewhat changed

According to these results, layers of Al₂O₃/Dy₂O₃ exhibit the bestresults with respect to their mechanical durability and the variabilityof the spectra after exposure to a moist-warm environment or to acidsand alkalis. In particular, they are more durable than layers of pureAl₂O₃. The layers comprising ytterbium oxide and gadolinium oxidecomponents are somewhat less durable than the layers of Al₂O₃/Dy₂O₃, butsignificantly better than layers of pure Al₂O₃.

1. A material for the production of optical layers of medium refractiveindex, consisting of aluminum oxide and at least two of gadoliniumoxide, dysprosium oxide and ytterbium oxide, which material isfunctional as a vapor-deposition material.
 2. The material according toclaim 1, consisting of aluminum oxide, gadolinium oxide, dysprosiumoxide and ytterbium oxide.
 3. The material according to claim 1,consisting of (i) aluminum oxide and (ii) two of gadolinium oxide,dysprosium oxide and ytterbium oxide in a ratio of i/ii from 1:99 to99:1 mol %.
 4. The material according to claim 1, consisting of 20 to 80mol % of aluminum oxide and 80 to 20 mol % of at least two of gadoliniumoxide, dysprosium oxide and ytterbium oxide.
 5. A process for thepreparation of a vapor-deposition material according to claim 1,comprising mixing aluminum oxide with at least two of gadolinium oxide,dysprosium oxide and ytterbium oxide, compressing or suspending themixture, shaping the mixture and subsequently sintering.
 6. The processaccording to claim 5, in which aluminum oxide and (ii) at least two ofgadolinium oxide, dysprosium oxide and ytterbium oxide are mixed in aratio of i/ii from 1:99 to 99:1 mol %.
 7. The process according to claim5, comprising mixing aluminum oxide with dysprosium oxide.
 8. Theprocess according to claim 5, in which the sintering is carried out at atemperature of 1300 to 1600° C. with inflow of air.
 9. The processaccording to claim 5, in which the sintering is carried out at atemperature of 1300 to 1600° C. under reduced pressure or under an inertgas.
 10. The process according to claim 5, in which the mixture isshaped into tablets, pellets, discs, truncated cones, grains, granules,rods or beads.
 11. A method for the production of optical layers ofmedium refractive index, comprising depositing by vapour deposition on aglass or plastic substrate, a material according to claim
 1. 12. Anoptical layer of medium refractive index on a glass or plasticsubstrate, having a refractive index of 1.6 to 1.9, said layercomprising a vapour-deposition material according to claim
 1. 13. Anoptical system comprising a glass or plastic substrate and thereon atleast one optical layer of medium refractive index having a refractiveindex of 1.6 to 1.9, comprising a material according to claim
 1. 14. Anoptical layer of medium refractive index on a glass or plasticsubstrate, having a refractive index of 1.6 to 1.9, said layercomprising a vapour-deposition material comprising aluminium oxide andat least one of gadolinium oxide, dysprosium oxide and ytterbium oxide.15. An optical system comprising a glass or plastic substrate andthereon at least one optical layer of medium refractive index having arefractive index of 1.6 to 1.9, said layer comprising avapour-deposition material comprising aluminium oxide and at least oneof gadolinium oxide, dysprosium oxide and ytterbium oxide.
 16. Anoptical layer of medium refractive index on a glass or plasticsubstrate, having a refractive index of 1.6 to 1.9, said layerconsisting of, as a vapour-deposition material, aluminium oxide and atleast one of gadolinium oxide, dysprosium oxide and ytterbium oxide. 17.A multilayered optical system comprising, as at least one optical layer,a medium refractive index layer having a refractive index of 1.6 to 1.9,said layer consisting of, as a vapour-deposition material, aluminiumoxide and at least one of gadolinium oxide, dysprosium oxide andytterbium oxide.